Traditionally, in a telephone switching center, time division multiplex lines are interconnected with analog trunks by means of an interface circuit known as a channel bank. Architecturally, a channel bank interconnects one duplex time division multiplex line with a plurality of analog 2-wire trunk lines by means of distinct receive and transmit paths terminated in a like plurality of 4-wire-to-2-wire hybrid transmission circuits. Along its receive path, a channel bank comprises circuits for decoding, demultiplexing, smoothing and level adjusting received PCM signals into continuous analog signals. Conversely, along its transmit path, analog signals appearing at the hybrid circuit are level adjusted, bandlimited, sampled, multiplexed and PCM coded for transmission onto the multiplex line.
Heretofore, much of channel bank testing has been achieved by looping or bridging together the transmit and receive paths on the digital PCM side of the channel bank apparatus. A prescribed analog test signal is applied to the transmit path, looped around in PCM form and recovered as a reconstituted analog signal at the output of the receive path. Comparisons between the prescribed and recovered analog signals for ascertaining various transmission criteria are carried out by means of analog supervisory devices. However, as is well known, such devices, being analog in nature, entail greater expense in comparison with digitally implemented methods of signal comparison and evaluation. Moreover, the use of these analog devices for testing channel banks is technologically incompatible with the use of both digital computer switching processors and digital switches which are proliferating in telephone switching centers.
Recently, in recognition of this problem, channel bank testing has been implemented by another approach. In U.S. Pat. No. 3,892,923 issued to G. Ranner on July 1, 1975, an arrangement is shown wherein the transmit and receive paths of a channel bank are looped or bridged together on the analog, rather than the digital side. A prescribed PCM test signal is applied at the input of the receive path wherealong it is decoded and demultiplexed. The resultant signal is looped around a dedicated analog path preconditioned for testing and is presented directly to the multiplexer and coding circuits of the transmit path. The reencoded PCM signals are recovered and compared, bit for bit, with the prescribed test code signal by means of conventional digital processing techniques.
Although the foregoing arrangement achieves some measure of digitally implemented testing with regard to the coder/decoder and multiplexer/demultiplexer circuitry within the channel bank, it is incapable of testing other portions of the receive and transmit paths of the channel banks. Specifically, the arrangement is deficient in that it cannot send the test code signals, between the demultiplexer and multiplexer, on a loop-around path that also includes the smoothing, band-limiting, and level adjusting subcircuits on the receive and transmit paths. Moreover, not only are such subcircuits excluded in the foregoing arrangement, but in order for bit-for-bit comparison between the prescribed and recovered signals to be successful, the arrangement requires that the loop-around path provided be preconditioned with a special smoothing circuit for testing purposes. Failure to test the aforementioned subcircuits may obscure the results of the test because normal acceptable levels of analog signal degradation may be experienced along the path connecting the aforementioned subcircuits and nevertheless give rise to a bit-for-bit mismatch due to analog-PCM signal resampling delay along the subcircuit path. The foregoing arrangement is further deficient in that it is unable to diagnostically specify the type of analog signal degradation experienced, e.g., amplitude loss, frequency-dependent distortion and background noise.
Accordingly, no prior art method is capable of accurately detecting and diagnosing various types of analog signal degradations in the entire channel bank by employing digitally implemented means.